The present invention relates generally to a tool for inserting and releasing a C-shaped spring clip into an elongate hollow structure. More particularly, this invention concerns a tool which is especially adapted for compressing a C-shaped spring clip, holding it in compressed position during insertion into a burnable poison rod of a fuel assembly of a nuclear reactor, and then releasing the spring clip for expansion in the desired position before withdrawing the tool from the rod.
Nuclear reactors of the type utilized in power generation incorporate a large number of fuel assemblies arranged in a suitable configuration. Each fuel assembly includes numerous fuel rods which are held in a fixed array by a "skeleton" comprising grids spaced at lengthwise intervals along the fuel rods, control rod guide thimbles interconnecting the grids and extending beyond the ends of the fuel rods, and top and bottom nozzles. Each fuel rod consists of a stack of fuel pellets within a tube, sealed by end plugs, which tube constitutes the primary containment boundary for the radioactive nuclear fuel.
In addition to fuel rods, such fuel assemblies also include control rods, secondary source rods, and burnable poison rods. The control rods are reciprocally movable within the guide thimbles to control the fission process over the life of the fuel assembly. The secondary source rods and burnable poison rods are not reciprocally movable, but are mounted in removable subassemblies which can be inserted into the fuel assembly. These two types of rods are generally used only at the beginning of the fuel cycle. The secondary source rods contain highly enriched material and thus provide additional reactivity to facilitate startup.
On the other hand, the burnable poison rods include neutron absorbing material, such as boron carbide, and function to enhance distribution of fuel during burnup. The neutron absorbing material of such rods is relatively brittle and expands during operation of the reactor, such that it is desirable to provide a mechanical restraint within the burnable poison rods to preclude any unnecessary movement of the absorber material during shipping and handling while permitting axial expansion later after being put in operation. It is therefore desirable to provide yieldable restraining devices in burnable poison rods.
Various types of springs and the like have been used for axial restraining purposes within a burnable poison rod. Helical coil springs often have been used, however, these have tended to be relatively long requiring too much space within the rods. Such springs are simply inserted into the tube before attachment of the adjacent end cap. More recently, C-shaped spring clips like that shown in U.S. Pat. No. 4,080,253 assigned to the assignee hereof have been utilized for this purpose. Such C-shaped clips are relatively compact and are adapted for positioning by means of an interference fit within the rods so as to provide sufficient frictional resistance to hold the pellets firmly in place during shipping and handling, while allowing controlled slippage to accomodate growth of the pellets during irradiation. Such clips are thus designed to provide predetermined frictional resistance with the inner walls of the rods, and are preferably compressed or deformed before insertion and positioning within the rods. Proper positioning of such spring clips therefore entails first circumferentially compressing the clips, inserting the compressed clips into the rods, and then releasing the clips in the desired positions. This can be both difficult and time consuming. Inserting the clips into the ends of the rods and then driving them inward to the proper positions is not desirable.
A tool has been developed for inserting such C-shaped spring clips into burnable poison rods. With this tool the clip is compressed by pinching before insertion into a slot in the end of a tool rod, and released by sliding an outer tube downward to move the clip off the slotted rod. This tool is thus characterized by a push-pull action which makes proper positioning of the clip difficult. It will be appreciated that such clips must be installed in engagement with the neutron absorber material, without gaps therebetween, to function effectively. Further, it is difficult to remove such clips with this type of tool, such as for repair of the rod. Heretofore, there has not been available a tool by which such clips could be conveniently compressed, inserted, and then released in the precise desired positions, or removed after insertion.
A need has thus arisen for a spring clip inserter tool of precise construction by which spring clips can be conveniently compressed, locked in compressed condition for insertion into the rod, and then positively released at the desired position before withdrawing the tool.